The present invention relates generally to electrolytic cells. More specifically, it relates to the use of a jumper switch system which permits electrical current to bypass at least one of a plurality of electrolytic cells connected in series to a power source to enable a cell to be removed from a bank or line of operating cells.
Electrolytic cells and, specifically, membrane cells, such as filter press membrane chlor-alkali cells are susceptible to damage when disconnecting one cell from a series of cells in a circuit. This damage primarily occurs to the catalytically active coatings that are employed on the electrode surfaces of these cells. Because of the high energy employed in electrolytic cells, jumper switches must be designed to avoid arcing and to eliminate reverse current flow during a cell's shutdown and removal.
The arcing problem is a two-fold problem, the first of which has been addressed by the use of vacuum switches, such as those manufactured by Westinghouse Corporation, that employ multiple interrupting modules either in pairs or singly to mechanically synchronize the opening of resistance modules in parallel with a number of normal current carrying modules. The interrupting modules are opened last to ensure that a multiple arc drop is achieved to produce a net arc voltage greater than the maximum cell voltage to counter the property of inductance which attempts to maintain current flow at a constant level throughout the cell circuit system. This approach solves the arcing problem which can shorten the life of the jumper switch for the switch manufacturers.
The second arcing problem concerns the safety of the operator during cell disconnecting operations. This problem is addressed by this invention. There is the potential, wherever an electromotive force (EMF) is generated to balance the cell back EMF which could cause a reverse current flow electrical current to arc across the area where an operator is disconnecting the intercell connecting links between bus bars of adjacent cells while removing one cell from an operating cell line.
Numerous approaches have been taken to counter the potentially damaging results stemming from the reverse current flow problem. Auxiliary circuits have been applied to cells to supply a DC cathodic protective current of low density to a cathode during periods of inoperation of a cell. A minimal current has been supplied to a cell below the decomposition voltage level during periods of cell inactivity to protect cells using ion exchange membranes. Another alternate approach has employed the addition of a reducing agent, such as sodium sulfite or urea, to the cathode compartment when the current flow in the cell is interrupted. The reducing agent reacts with any sodium hypochlorite present in the electrolyte in the cathode compartment to prevent the deterioration of the transition metal coating on the surface of the cathode or any transition metal in the cathode itself. Still another approach has employed the use of a cell protective current between a conductor and the electrode in the cell during cell shutdowns or disconnections to prevent the passage of reverse currents through the cell.
A recent approach has employed the use of a short circuiting unit or jumper switch that has a resistor and a switch combination connected in parallel to at least one of the cells in an electrolytic cell line. A switch is closed to provide a closed loop so that current will flow through the cell in the same direction as current flows during electrolysis, but this current flow is smaller than the normal current flow during electrolysis. This system almost immediately dramatically reduces the reverse current flow after the closing of the bypass circuit switch, but there is still reverse current flow. After a finite period of time the reverse current flowing in the direction opposite to the normal current flow approaches zero.
However, all of the prior approaches have either required the use of expensive additional equipment to generate protective auxiliary current flows, the use of expensive equipment such as rectifiers, or have not completely eliminated the reverse current or back EMF flow that causes the catalytic coating on the cathode surface or the cathode itself to begin to oxidize and become, for example, a chlorine consuming instead of a chlorine generating surface. Once such damage occurs to the cathode, the cathode voltage consumption can increase from about 10 to about 20 millivolts and can shorten the economic life of a cathode after shutdown with a jumper switch.
The foregoing problems are solved in the design of the jumper switch system of the present invention.